1. Field of the Invention
The present invention relates to a powder feeding apparatus, a powder feeding method and a powder pressing apparatus, and more specifically to a powder feeding apparatus and a powder feeding method used for feeding a powder into a cavity of a tooling when pressing the powder for manufacture of a rare-earth magnet, and a powder pressing apparatus comprising such a powder feeding apparatus.
2. Description of the Related Art
In manufacture of a rare-earth magnet for example, a raw material rare-earth alloy is first milled into a powder, then pressed into a compact, sintered, aged, surface-treated and so on before the final sintered magnet is produced. In the above process, uniformity and accuracy in the amount of feeding the powder in the pressing step has a significant meaning in coercive force, remanence and dimensional accuracy of the product. If there is substantial inconsistency in the amount of feeding, there results correspondingly large inconsistency in coercive force, remanence and dimensional accuracy. In an attempt to overcome the problem therefore, there has been a number of powder feeding apparatuses proposed.
For example, as a powder feeding apparatus of this kind, a powder feeding apparatus as shown in FIGS. 17.about.19 is proposed in the Japanese Patent Application No. 11-364889.
The powder feeding apparatus 1 shown in the above figures is used in manufacture of a rare-earth magnet. The powder feeding apparatus 1 includes a feeder box 3 having a bottom face formed with an opening 2 and movable above an opening of a cavity formed in a tooling. Inside the feeder box 3, there are provided a plurality of rod members 4 for horizontal movement at least at a bottom portion of the feeder box 3. The rod members 4 are moved by a cylinder mechanism 5. The parallel movement of the rod members 4 provides a pushing action, making possible to supply a predetermined volume of a powder 6 into the cavity of the tooling.
Another related art is disclosed in the Japanese Patent Laid-Open No. 61-147802.
According to this disclosure, a magnetic powder is put in a feeder cup having a bottom portion attached with a metal net. The feeder cup is vigorously vibrated by a solenoid, thereby sieving the magnetic powder by the metal net into a granular form when fed.
However, according to the powder feeding apparatus 1 disclosed in the Japanese Patent Application No. 11-364889, the parallel movement of the rod members 4 creates hollows and coagulated drifts in the powder 6, causing inconsistency in density of the powder 6 supplied in the cavity. Further, since the apparatus is to feed a predetermined volume, even if stable supply is achieved in the volume, there is sometimes difference in the weight of the feed, resulting in inconsistency in the weight of the compact.
Further, when the feeder box 3 is withdrawn from above the cavity, the bottom face of the feeder box 3 drags the powder 6 on top of the cavity. This also causes inconsistency in the amount of the powder supply, resulting especially in inconsistency in feeding density near the cavity top surface, sometimes leaving undulation in the surface.
Still further, since the powder 6 has a small grain diameter of 1 .mu.m.about.5 .mu.m, the powder 6 is unavoidably left on the die. Especially, when a magnetic field is applied, the powder 6 left on the die gathers around the cavity, but the powder 6 scattered on the die has already been oxidized by contact with air. This oxidized powder 6 is wiped into the cavity by a front face of the feeder box 3 when the feeder box 3 is moved onto the cavity. When sintered, the oxidized powder 6 increases oxygen inclusion in the magnet, deteriorating magnetic properties.
Further, if the pressing is performed without sufficient amount of the powder 6 supplied, a resulting compact becomes more apt to crack, or density difference in the compact results in dimensional inconsistency after the sintering. This is especially problematic when forming a thin compact. The related art disclosed in the Japanese Patent Laid-Open No. 61-147802 also uses the wiping technique for the feeding. Thus, even with the provision of the metal net, there are the same problems as described above.